1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary battery having lithium or a material capable of absorbing and releasing lithium as a negative electrode active material and using a non-aqueous electrolyte conductive to lithium ions, and particularly to an improvement of the positive electrode thereof.
2. Descrition of the Related Art
In recent years, along with a marked proliferation of portable electronic equipment and communications devices, various devices requiring batteries with high current outputs as power supplies have appeared, and from the points of view of economy and downsizing and downweighting of these devices, high energy density secondary batteries are being demanded. As a result of this, research and development of non-aqueous electrolyte secondary batteries having high voltages and high energy densities have been being carried out intensively, and some such batteries have been put into practical use.
In the past, as positive electrode active materials used in this kind of secondary battery, various materials, such as metal chalcogenides such as TiS.sub.2, MoS.sub.2 and NbSe.sub.3 and metal oxides such as MnO.sub.2, MoO.sub.3, V.sub.2 O.sub.5, Li.sub.x CoO.sub.2, Li.sub.x NiO.sub.2 and Li.sub.x Mn.sub.2 O.sub.4, have been proposed.
Among these positive electrode active materials, lithium transition metal oxides Li.sub.x M.sub.y O.sub.2 (x.ltoreq.1, y.apprxeq.1) such as Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2 having .alpha. -NaCrO.sub.2 -type layer-like structures undergo a battery reaction with a lithium negative electrode expressed by the following formula (1): EQU Li.sub.x1 M.sub.y O.sub.2 .revreaction.Li.sub.x1-x2 M.sub.y O.sub.2 +x.sub.2 Li.sup.+ +x.sub.2 e.sup.- ( 1)
(where x.sub.1 is the amount x of Li before charging, x.sub.2 is the amount x of Li after charging, 0&lt;x.sub.1 .ltoreq.1 and 0&lt;x.sub.1 -x.sub.2 .ltoreq.1), and the operating voltage thereof is a high voltage of over 4V. Also, when by charging in the range x=0 to 1 Li ions are made able to deintercalate and intercalate, these materials offer high theoretical energy densities of over 1100 whr/kg, and are therefore considered promising (Japanese Laid-Open Patent Publication No. S.55-136131).
However, there has been the problem that in a battery having lithium or a material capable of absorbing and releasing lithium as a negative electrode active material and a conventional Li.sub.x M.sub.y O.sub.2 of the kind described above as a positive electrode active material, at practical charge/discharge voltages and current densities the effective charge/discharge capacity is small, at less than 50% of the theoretical capacity, and the greater the current is the smaller the capacity becomes. Also, there has been the problem that the reduction in operating voltage during discharging due to polarization is large.
The reason for this is that because along with Li ions being drawn out (deintercalation) of the Li.sub.x M.sub.y O.sub.2 of the positive electrode by charging the electrode potential of the Li.sub.x M.sub.y O.sub.2 increases markedly and also because polarization due to the fact that the Li ion conductivity and electron conductivity are low is great the charging voltage markedly increases, and consequently at practically stable voltages below the decomposition potentials (about 4 to 4.5V with respect to metallic lithium) of electrolytes, which will be further discussed later, which can be used in these batteries and the oxidization potential of the battery case and collectors and the like, the chargeable capacity falls markedly. In particular, when the amount x of Li in the Li.sub.x M.sub.y O.sub.2 is in the region below about 0.6 the increase in potential is marked and at practical charging voltages and current densities this region essentially cannot be used.
Also, there has been the problem that because during charging crystal lattices undergo structural changes such as expansion and contraction and phase change, the crystal structure is destroyed by repeated charging and discharging and the charge/discharge capacity gradually falls.
To resolve these problems, the use of [1] a composite oxide A.sub.x M.sub.y N.sub.z O.sub.2 (where 0.05.ltoreq.x.ltoreq.1.10, 0.85.ltoreq.y.ltoreq.1.0, 0.001.ltoreq.z.ltoreq.0.10; Japanese Laid-Open Patent Publication No. S.62-90863) of a metal N such as Al, In or Sn and a transition metal M and an alkali metal A, and [2] a composite oxide Li.sub.y Ni.sub.x Co.sub.1-x O.sub.2 (where 0&lt;x.ltoreq.0.75, y.ltoreq.1; Japanese Laid-Open Patent Publication No. S.63-299056) and the like has been proposed.
By the use of these composite oxides, the charge/discharge characteristics are considerably improved; however, they still fall far short of theoretical capacities, and in particular charge/discharge capacities at the high currents necessary in practice have been low and inadequate.
To solve the above kinds of problem, the present inventors have already proposed the use of [3] a composite oxide Li.sub.x M.sub.y L.sub.z O.sub.2 (where 0&lt;x.ltoreq.1.15, 0.85.ltoreq.y+z.ltoreq.1.3, 0&lt;z; Japanese Laid-Open Patent Publication No. H.5-54889) of one or more elements L chosen from among the periodic table IIIA, IVA and VA group non-metal elements and semi-metal elements, alkaline earth metal elements and metals selected from the group consisting of Zn, Cu, Ti, with a transition metal M and lithium Li. By the use of this kind of composite oxide, charge/discharge characteristics were further improved; however, there has still been room for improvement of charge/discharge characteristics at high current densities as compared to theoretical capacities.
Also, as the transition metal M, the charge/discharge characteristics are particularly good in the cases of Co and Ni and therefore the use of these metals is preferable, but in the case of Co the potential is essentially high, and especially in the charging region above 50% of the theoretical capacity the potential rises markedly and consequently there is decomposition of the electrolyte and change in the crystal structure, and it has in practice been very difficult to obtain a charge/discharge capacity of over 60% of the theoretical capacity stably. Furthermore, there is the drawback that Co resources are limited and its cost is high. In the case of Ni, on the other hand, charging and discharging to over 80% of the theoretical capacity is possible; however, there are the drawbacks that reduction in charge/discharge capacity (cycle deterioration) caused by repeated charging and discharging is great and deterioration with time is also great. Also, there has been the shortcoming that when thermal synthesis is carried out, which will be further discussed later, synthesis in air is difficult because cubic crystal structures of low charging/discharging performance tend to be produced, and it is necessary to carry out synthesis in an oxygen atmosphere and finely control the atmosphere and the temperature, which makes the manufacturing process complicated and increases costs.